There are many applications of carbon nanotubes (CNTs) because of their unique mechanical, physical, electrical, chemical and biological properties. For example, ultra low resistance conductors, semiconductors, highly efficient electron emitters, ultra-strong lightweight fibers for structural applications, and lasers can all be manufactured by using CNTs. For reviews of CNT technology, properties and applications, see Baughman et al., “Carbon Nanotubes—the Route Toward Applications”, Science, volume 297, pages 787-792 (2002); Michael J. O'Connell (Editor) “Carbon Nanotubes—Properties and Applications”, CRC Taylor & Francis, New York (2006); and Yury Gogotsi (Editor) “Nanomaterials Handbook”, CRC Taylor & Francis, New York (2006).
A great deal of research effort has been directed toward the development of small dimensional inexpensive gas sensing devices for applications including monitoring and controlling environmental pollution; providing small, low-power, rapid and sensitive tools for process and quality control in industrial applications; and implementing or improving detection and quantification of harmful gases.
In many industries, gases have become increasingly important as raw materials and it has thereby become very important to develop highly sensitive gas detectors. Such devices should allow continuous monitoring of the concentration of particular gases in the environment in a quantitative and selective way. However, many of these efforts have not yet reached commercial viability because of problems associated with the sensor technologies applied to gas-sensing micro-systems. Inaccuracies and inherent characteristics of the sensors themselves have made it difficult to produce fast, reliable and low-maintenance sensing systems comparable to other micro-sensor technologies that have grown into widespread use commercially.
The practical application of environmental monitoring requires developing sensing devices that are smaller and cheaper than the analytical instruments currently used. Much of the research on gas sensors to date has been carried out using either thick-film or thin-film metal oxide semiconductor sensors. Development of such sensors may have resulted in devices with reasonable sensitivities. For environmental purposes, however, greater sensitivities are required.
Among the gaseous species to be observed in air as contaminants (polluting gases) are nitrous oxide (NO), nitrogen dioxide (NO2), carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), hydrogen sulfide (H2S), sulfur dioxide (SO2), ozone (O3), ammonia (NH3), and organic gases such as methane (CH4), propane (C3H8), liquid petroleum gas (LPG) organic vapors (ethanol, formaldehyde) and the like.
For detection and quantification of carbon dioxide in gas mixtures, there are two types of conventional sensors, i.e. infrared spectroscopy (IR) based sensors and electrical resistance based metal oxide semiconductor (MOS) sensors. The IR sensors take advantage of the large IR stretching band for C═O functionality at 2349 cm−1. Although commercially available portable IR sensors exist, this approach is still limited by its power consumption, size and cost. The MOS sensors utilize the change of electrical resistance of a semiconductor film in the presence of carbon dioxide. These sensors are also commercially available. However, since such sensors operate at high temperatures, they increase the power consumption.
There is a need for new or improved sensors that can be used for fuel cell equipment in monitoring the hydrogen concentration in the fuel stream (thereby its purity) and detection of equipment leakages.
Detection and quantification of ethanol is also becoming important for a variety of purposes including ethanol production, chemical processing, fuel processing and use, societal applications, and physiological research on alcoholism. A large number of commercial ethanol measurement systems are available for several of these applications. However, in general, these systems are designed exclusively for vapor-phase measurements, operate at relatively high power levels, are bulky, and possess functionality that is more limited than required for a number of applications. For the most precise measurements, high performance liquid chromatography (HPLC) and infrared spectroscopy (IR) can be used for ethanol concentration measurements. However, these are expensive and involve large equipment. For portable detection, smaller handheld devices such as breathalyzers are used for measurements that are proportional to blood alcohol concentration (BAC). Breathalyzer devices acquire ethanol from exhaled breath and require direct and intimate exhalation into the apparatus. Different versions of these devices have been integrated into some models of various commercial vehicles. The driver is required to breathe into a special mouthpiece to measure the level of alcohol in the breath, and a computer decides whether or not to allow the engine to start. In all these cases the measured breath alcohol is indiscreet and can be difficult to correlate to the blood alcohol concentration, since there can be a lot of variation in the breath collection method. Moreover, while semiconductor metal oxides such as SnO2 and ZnO have typically been employed for alcohol sensing, these materials operate at elevated temperatures (>150° C.) and are sensitive to adsorption of other gaseous species apart from ethanol such as gasoline, CO, hydrocarbons and hydrogen which interfere with the alcohol measurements.
With the increasing demand for superior but inexpensive gas sensors of higher sensitivity and greater selectivity, intense efforts are being made to find more suitable materials with the required surface and bulk properties for use in gas sensors.
The carbon nanotubes (CNTs) are investigated as materials suitable for manufacturing such sensors. For example, Robinson et al. in “Improved Chemical Detection Using Single-Walled Carbon Nanotube Network Capacitors” Sensors and Actuators A, volume 135, pages 309 to 314 (2007); Varghese et al. in “Gas Sensing Characteristics of Multi-wall Carbon Nanotubes” Sensors and Actuators B, volume 81, pages 32 to 41 (2001); Valentini et al. in “Highly Sensitive and Selective Sensors Based on Carbon Nanotubes Thin Films for Molecular Detection” Diamond and Related Materials, volume 13, pages 1301 to 1305 (2004); Snow et al. in “Chemical Vapor Detection Using Single-Walled Carbon Nanotubes” Chemical Society Reviews, volume 35, pages 790 to 798 (2006); and Star et al. in “Gas Sensor Array Based on Metal-Decorated Carbon Nanotubes” Journal Physical Chemistry B, volume 110, pages 21014 to 21020 (2006) have described the detection and quantification of gaseous species in gas mixtures by sensors manufactured by using CNTs.
Sensors arrays have also been proposed for detection or determination of concentration of more than one analyte, e.g. by Lu et al., in “A carbon nanotube sensor array for sensitive gas discrimination using principal component analysis”, J. Electrochem. Chem., volume 593, pages 105 to 110 (2006); by Qi et al. in “Toward Large Arrays of Multiplex Functionalized Carbon Nanotube Sensors for Highly Sensitive and Selective Molecular Detection”, Nano Lett., volume 3, pages 347 to 351 (2003); and by Graf et al., in “Smart single-chip CMOS microhotplate array for metal-oxide-based gas sensors” 12th International Conference on Transducers, Solid-State Sensors, Actuators and Microsystems, volume 1, pages 123 to 126 (2003).
Methods of preparation of variety of sensors and sensor arrays have also been proposed, for example, by Eranna et al., in “Oxide Materials for Development of Integrated Gas Sensors—A Comprehensive Review”, Critical Reviews in Solid State and Materials Sciences, volume 29, pages 111 to 188 (2004); and by Sabate et al., in “Multisensor Chip for Gas Concentration Monitoring in a Flowing Gas Mixture”, Sensors and Actuators B, volume 107, pages 688 to 684 (2005).
In summary, in all these applications, there is a high demand for improved sensitivity, accuracy, reliability, selectivity and stability beyond what is currently offered by commercially available sensors. There exists a need for new or improved sensor devices for detecting analyte gas.